What I like most about Starcraft's space marines is that they nail the dystopian nature of human society in their universe far better than other sci-fi dystopias ever do.
When you hear the word "space marine", you automatically begin to assume the best of the best, the cream of the crop. The Adeptus Astartes of the Imperium of Man. The Dark Troopers and Zero-G Space Troopers of the Galactic Empire. The UNSC's Spartans. These are units that are usually reserved for the toughest of missions, units that get the best treatment, equipment, and honors, units that represent the best of their faction.
In Starcraft, however, the Space Marine is not a mighty creature. Instead, he's almost the smallest runt on the battlefield, dwarfing only the Zergling. And instead of being the best of the best, they're just random idiots and criminals drafted by the state, brainwashed by its academic institutions, and shoved inside power armor, all the while, the marine is doped up on drugs that increase their speed and motor functions at the cost of damage to the body. It really goes to show how dystopian the Terran Confederacy and its successor state, the Dominion, were-that they were ready to throw brainwashed criminals into the battlefield with power armor and large guns, encouraging them to take drugs that would damage their bodies just to increase combat effectiveness.
Of course, the UED doesn't share the same idea, as they have medics whose primary function is to heal the marines and keep them at tip-top shape, but prior to the UED's invasion of the Koprulu Sector, the marines of the Confederacy and the Dominion were little more than walking meatshields for the mechanized units and the air force; distractions that can soak up enemy fire so that the heavier units can blow the enemy to bits.
It's a clever reverse of the Space Marine power fantasy, where instead of being the best of the best, they're the bottom bitch of the food chain, and only through numbers and tactics can they win against practically anything that isn't a Zergling. Not to mention that it makes things even more grim than they are in other works, where the guy with power armor and big guns that shoot depleted uranium are barely enough to hold off the common mooks of the other species, let alone their heavy-hitters. Whereas in works like 40K or Halo, even if you're a lowly guardsman/UNSC trooper, if you can hide behind a Primaris Marine or a Spartan-2, you'll usually come out OK against an enemy force.